Aug 272015
 

Humidity sensor circuit using the 555 timer

This simple humidity sensor circuit, shows us the humidity level that land under test has. To achieve the goal, a couple of probes are used. These probes are placed as shown on the diagram, with their ends buried in the ground to be tested.

The circuit uses two LEDs, which alternate their illumination at a rate which depends on the degree of soil moisture. This is very useful to know when you need to water the plants. The two extreme cases are:

– With a dry ground, only one LED lights up
– When the ground is wet, the speed of the alternating illumination of LEDs is high.

Humidity sensor circuit using the 555 timer

How the humidity sensor circuit works?

This humidity sensor circuit uses the 555 integrated circuit (timer) configured as astable multivibrator. For the circuit to oscillate, two resistors are needed:

  • R1: in the location shown in the diagram.
  • R4: that is not shown in the diagram, but in this case is replaced by the resistance of the ground under test. (obtained using probes).

Depending on the moisture of the ground, its resistance (R4) will vary, and hence the oscillation frequency of the 555 timer. This frequency is displayed on the LED. LEDs can be the same color, but in this case we use one green LED and one red LED to achieve a more noticeable display.

This circuit becomes portable, if you use a 9-volt battery.

List of circuit components for the humidity sensor circuit

  • 1  555 Timer (CI)
  • 3  1K resistors (R1,R2,R3)
  • 1  0.1 uF capacitor (C2)
  • 1  10uF electrolytic capacitor (C1)
  • 2  LEDs. (1 red , 1 green) (D1,D2)
  • 1  9 volt battery.

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  2 Responses to “Humidity sensor circuit using the 555 timer”

  1. In the diagram of humidity sensor, R2 is clearly shown whereas it is mentioned that R2 is not shown! . Please clarify this.

    • Hi Alex

      I already made the corrections to the article, I hope it is understood well now, R4 is the resistance of the ground under test.

      Thank You. Regards

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